Ozone Continues to Increase in East Asia Despite Decreasing NO2: Causes and Abatements

Space-borne ozone (O3 ) measurements have indicated consistent positive trends across the entire Asia–Pacific region despite the considerable reduction of NOx since 2000s. The rate of increase in O3 derived from lower free tropospheric column measurements was observed to be 0.21 ± 0.05 DU/decade during 2005–2018. Our space-borne-based diagnosis of the nonlinear photochemical formation regimes, NOx -limited and NOx -saturated, show that O3 chemistry is undergoing a transitional process to the NOx -limited regime throughout most of the Asian region. Nevertheless, NOx -saturated conditions persist at present in and over eight major megacities. These NOx -saturated conditions in megacities contribute to the increased O3 due to NOx reduction, which could also affect the enhanced O3 concentrations throughout the Asia–Pacific region via long-range transport. This indicates that VOC limits along with NOx reductions are needed in megacities in the short term to reduce O3 levels. Moreover, NOx saturation in major megacities will continue until 2025, according to the forecast emission scenarios from the Intergovernmental Panel on Climate Change (IPCC). These scenarios gradually shift nearly all cities to the NOx -limited regime by 2050 with the exception of few cities under IPCC RCP8.5. Thus, continued reductions in NOx will be a key factor in reducing O3 in the long term.